Wireless handsets are increasingly required to operate in multiple modes such as Global System for Mobile communications (“GSM”)/Global Packet Radio Service (“GPRS”), Enhanced Data rates for Global Evolution (“EDGE”), Code Division Multiple Access (“CDMA”), Wideband Code Division Multiple Access (“WCDMA”)/High-Speed Packet Access (“HSPA”), and Long Term Evolution (“LTE”). The handsets are also increasingly required to operate in multiple frequency bands such as 700 Megahertz (“MHz”), 800 MHz, 900 MHz, 1700 MHz, 1800 MHz, 1900 MHz, 2100 MHz, and 2600 MHz bands. Multi-mode and multi-band handset models currently in use contain a separate dedicated power amplifier for each individual mode and band of operation.
The purpose for using dedicated power amplifier circuits for each band and mode is that the input and output match of the circuit must be optimized to achieve the best linearity and/or efficiency for the given mode or band of operation. Although the main transistor of the amplifier circuit is inherently broadband, the bandwidth of the amplifier circuit is typically made narrower by the input and output matching circuits. Therefore, to achieve acceptable linearity and efficiency, power amplifier circuits using fixed matching networks respectively tuned for the different bands and modes of operation are used in a wireless terminal. Using fixed matching networks, a semiconductor power transistor device can only efficiently transmit RF signals in a single mode and a single band.
No viable solution exists for a single power amplifier (“PA”) to cover both GSM and CDMA modes because, in the GSM system, the PA transistor operates in saturated region, a much narrower bandwidth signal and time slotted; whereas in the CDMA system, the PA transistor has to operate at a more linear region in continuous time. This difference leads to very different impedance matching solutions at the output of a PA device. In a fixed impedance system design, such as used 50 ohm system, a fixed matching network cannot satisfy both modes simultaneously.
In terms of frequency coverage, a single power amplifier circuit typically can only cover either a low band (800 MHz/900 MHz), or a high band (1700 MHz/1800 MHz/1900 MHz), or a UMTS band (2100 MHz). The load impedance presented at the output of the power amplifier transistor can be quite different at various operating frequencies and a single fixed matching network cannot provide optimum matching for all potential frequency bands simultaneously. Therefore, multiple PAs are required in the multi-mode multi-band handset.
The ever decreasing form factor and ever increasing functionality demanded of wireless terminals creates conflicting challenges on front-end devices like number of PAs that can be installed. Currently, the handset board space limits the number of PAs to no more than four on even the most complicated handset units. Despite much effort within the industry to aggressively reduce device sizes, the fundamental physics and fabrication challenges prohibit the ability to reduce the device sizes at a pace necessary to accommodate the number of added functions within the handset unit. Not only do additional PA devices require more board space, the peripheral passive components around each PA device also require board space and increase proportionately with the number of PAs used.
Therefore, there is a need in the art for an improved power amplification method. In particular, there is a need for a power amplifier that is capable of amplifying multiple modes and multiple bands while minimizing board space requirements.